The Problem

You, like me once, just upgraded to Xcode 4 and now you can't compile blender... This happens because the blender compiler (BTW: I use scons to compile) uses the old sdk 10.5 and/or 10.4u, which is not available on Xcode 4.

If you need help getting the initial setup to compile blender, view this video done by the awesome people at blender cookie.